ece453 – introduction to computer networks
DESCRIPTION
ECE453 – Introduction to Computer Networks. Lecture 16 – Application Layer (DNS, E-mail, Web, FTP). application transport network data link physical. application transport network data link physical. application transport network data link physical. - PowerPoint PPT PresentationTRANSCRIPT
ECE453 – Introduction to Computer Networks
Lecture 16 – Application Layer(DNS, E-mail, Web, FTP)
Applications and Application Level Protocols
The three concepts Service model Protocol Interface
Network application is more than application level protocols Client site Server site Application level protocol
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
Client/Server ParadigmClient
Initiates contact with server (speak first)
Typically request service from server
Question: identify who is/implements client in
Web? Email?
Server Provides requested service to
clients Question: identify who
is/implements the server counterpart in
Web? Email?
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
request
reply
Which Transport Service Does Application Need? - Parameters
Data Loss Loss-tolerant applications, e.g. audio/video other app such as file transfer, telnet requires
100% reliable transmission
Bandwidth Bandwidth-sensitive applications, such as
multimedia, require a maximum amount of bandwidth
Elastic applications: can use whatever bandwidth available
Timing Some apps such as internet telephone requires
“low delay” to be effective
Transport Service Required By Common Applications
Application
file transfere-mail
Web documentsreal-time audio/video
stored audio/videointeractive games
financial apps
Data loss
no lossno lossloss-tolerantloss-tolerant
loss-tolerantloss-tolerantno loss
Bandwidth
elasticelasticelasticaudio: 5Kb-1Mbvideo:10Kb-5Mbsame as above few Kbps upelastic
Time Sensitive
nononoyes, 100’s msec
yes, few secsyes, 100’s msecyes and no
Internet Apps and Their Transport Layer Protocols
Application
e-mailremote terminal access
Web file transfer
streaming multimedia
remote file serverInternet telephony
Applicationlayer protocol
smtp [RFC 821]telnet [RFC 854]http [RFC 2068]ftp [RFC 959]proprietary(e.g. RealNetworks)NFSProprietary (private)(e.g., Vocaltec)
Underlyingtransport protocol
TCPTCPTCPTCPTCP or UDP
TCP or UDPtypically UDP
DNS – Domain Name System
Name: panda.ece.utk.edu
IP: 160.36.30.108
DNS: Mapping Name To Address
Name: panda.ece.utk.edu is used by humanIP address: a 32-bit numerical value used by machineDNS A distributed database, implemented by a
hierarchy of name servers Application level protocols used by hosts,
routers and name servers Internet intelligence is on the edge
DNS Name Space
biz, info, name, proaero, coop, museum ICANN
http://www.icann.org/
DNS – Why Not Centric?
Single point of failureTraffic volumeDistant name server means slow responseScalability
History: ARPANET begins with a single hosts.txt.
DNS – Hierarchical View
Local DNS serverAuthoritative DNS serverRoot DNS server
•
DNS: Where Are Root Servers?
•
DNS: An Example
Case: Root server knows authoritative DNS server
requesting hostpanda.ece.utk.edu
worf.mcnc.org
root name server
local name serverdns.ece.utk.edu
1
23
45
6
Authoritative DNS serverdns.mcnc.org
DNS: An Example
Case: Root server doesn’t know immediate authoritative DNS server, but know the intermediate one
requesting hostpanda.ece.utk.edu
people.anr.mcnc.org
root name server
local name serverdns.ece.utk.edu
1
23
67
8
Intermediate serverdns.mcnc.org
Authoritative serverdns.anr.mcnc.org
5
4
DNS: Iterated Query
Root server replies with the name of intermediate serverIterated query vs. recursive query requesting host
panda.ece.utk.edupeople.anr.mcnc.org
root name server
local name serverdns.ece.utk.edu
1
23
8
Intermediate serverdns.mcnc.org
Authoritative serverdns.anr.mcnc.org
6
5
4
7
Iterated query
DNS Caching
Once (any) server learns new mapping, it caches itThe cache will expire after some timeUpdate/notify mechanism is defined by IETF RFC 2136
DNS Server and ServiceRunning on top of UDPPort number: 53Frequently used by other applications such as SMTP, FTP, HTTPImportant services
Host aliasing Mail server aliasing Load distribution (DNS rotation)
User utilities: dig, http://www.netliner.com/dig.htmlMore DNS information: see DNS NET http://www.dns.net/dnsrd/docs/
*DNS Resource RecordDNS: distributed database storing resource
records (RR)RR format: (domain_name, ttl, class, type, value)
*DNS: Message FormatDNS protocol : query and reply messages, both
with same message format
message header• identification: 16 bit # for query, reply to query uses same #• flags:
• query or reply• recursion desired• recursion available• reply is authoritative
*DNS Protocol Message
Name, type fields for a query
RRs in reponseto query
records forauthoritative servers
additional “helpful”info that may be used
Electronic MailThree major
components: user agents mail servers simple mail transfer
protocol: smtp
User Agent “mail reader” composing, editing,
reading mail messages e.g., Eudora, Outlook, elm,
Netscape Messenger outgoing, incoming
messages stored on server
user mailbox
outgoing message queue
mailserver
useragent
useragent
useragent
mailserver
useragent
useragent
mailserver
useragent
SMTP
SMTP
SMTP
Electronic Mail: Mail Server
mailbox contains incoming messages (yet to be read) for usermessage queue of outgoing (to be sent) mail messagesCommon Mail Server: Sendmail MS Exchange
SMTP: RFC 821
Use TCP for reliable transfer, use port number 25Message must be 7-bit ASCII
[hqi@aicip hqi]$ telnet panda.ece.utk.edu 25Trying 160.36.30.108...Connected to panda.ece.utk.edu.Escape character is '^]'.220 panda.ece.utk.edu ESMTP Sendmail 8.11.6/8.11.6; Thu, 21 Nov 2002 09:54:04 -0500HELO panda.ece.utk.edu250 panda.ece.utk.edu Hello pegasus.ece.utk.edu [160.36.30.110], pleased to meet youMAIL FROM: <[email protected]>250 2.1.0 <[email protected]>... Sender okRCPT TO: <[email protected]>250 2.1.5 <[email protected]>... Recipient okDATA354 Enter mail, end with "." on a line by itselfdo you like ketchup?how about pickles?.250 2.0.0 gALEt5U25932 Message accepted for deliveryQUIT221 2.0.0 panda.ece.utk.edu closing connectionConnection closed by foreign host.
Mail Access Protocol – Final Delivery
SMTP: delivery/storage to receiver’s server Mail access protocol: retrieval from server
– POP: Post Office Protocol [RFC 1939] (port 110)o authorization (agent <-->server) and download o Does not maintain state across POP sessionso Cannot manipulate emails at the server side
– IMAP: Internet Mail Access Protocol [RFC 1730]o more features (more complex)o manipulation of stored msgs on servero Maintain state for the user
– HTTP: Hotmail , Yahoo! Mail, etc.– Slow
useragent
sender’s mail server
useragent
SMTP SMTP POP3,IMAP, or
HTTPreceiver’s mail server
http
Summary
Application Client Server Protocol
What type of service
Through what interface
Which port
DNS Aliasing vs. load
distribution “nslookup” and
“dig”
Email SMTP Mail access protocol
POP3 IMAP HTTP
Web: TerminologyWeb page
Consists of “objects” Addressed by “url”
(universal resource locator)
Most of web page One base web page Several referenced
“objects”
URL has two components A host name and a path http://panda.ece.utk.edu/
~hqi/teaching.html
Web client Netscape
communicator Mozilla Microsoft IE browser
Web server Apache Microsoft Internet
Information Server (IIS)
Web: the http Protocol
Web application layer protocol: a hyper text transfer protocol, httpDefined by
HTTP 1.0, RFC 1945 HTTP 1.1, RFC 2068
Client/Server Mode Client: browser asks for
objects, and display it Request Display
Server: provide objects in response to requests
PC runningExplorer
Server running
NCSA Webserver
Mac runningNavigator
http request
http re
quest
http response
http re
sponse
Web: http Operation Flow
HTTP utilizes TCP transport servicesHTTP client initiates TCP connection (create socket) to server, at port 80Server accepts this connection from clientHTTP messages (defined by HTTP protocol) are exchanged between http client and http serverTCP connection closed
HTTP is stateless Server doesn’t
maintain the state of past requests ‘back’?
HTTP ExampleSuppose user enters URL
www.someSchool.edu/someDepartment/home.index
1a. http client initiates TCP connection to http server (process) at www.someSchool.edu. Port 80 is default for http server.
2. http client sends http request message (containing URL) into TCP connection socket (plus another acknowledge message)
1b. http server at host www.someSchool.edu waiting for TCP connection at port 80. “accepts” connection, notifying client
3. http server receives request message, forms response message containing requested object (someDepartment/home.index), sends message into socket
(contains text, references to 10
jpeg images)
time
HTTP Example (cont’d)
5. http client receives response message containing html file, displays html. Parsing html file, finds 10 referenced jpeg objects
6. Steps 1-5 repeated for each of 10 jpeg objects
4. http server closes TCP connection.
time
• Two RTT (Round-trip time)• Slow start• Place burden on the Web server
HTTP: Persistent and Non-persistent Connection
Non-persistent HTTP 1.0 Server parses request, responds, then
closes TCP connection Each object requires 2 RTT Each object suffers slow start
Persistent HTTP 1.1 On the same TCP connection, server parses
request, responds, and parses new requests
SMTP vs. HTTP
HTTP: Direct connection, no intermediate mail serversBoth use persistent connectionHTTP is a pull protocol, while SMTP is a push protocolSMTP: 7-bit ASCII format, message ended with a line consisting of only a period
*HTTP Message Format
two types of http messages: request, responsehttp request message:
ASCII (human-readable format)
GET /somedir/page.html HTTP/1.0 User-agent: Mozilla/4.0 Accept: text/html,image/gif,image/jpeg Accept-language:fr
(extra carriage return, line feed)
request line(GET, POST,
HEAD commands)
header lines
Carriage return, line feed
indicates end of message
*HTTP Request: General Format
*HTTP Message Format: Response
HTTP/1.0 200 OK Date: Thu, 06 Aug 1998 12:00:15 GMT Server: Apache/1.3.0 (Unix) Last-Modified: Mon, 22 Jun 1998 …... Content-Length: 6821 Content-Type: text/html data data data data data ...
status line(protocol
status codestatus phrase)
header lines
data, e.g., requestedhtml file
*HTTP Response: Status Code
200 OK request succeeded, requested object later in this
message
301 Moved Permanently requested object moved, new location specified
later in this message (Location:)
400 Bad Request request message not understood by server
404 Not Found requested document not found on this server
505 HTTP Version Not Supported
Try Out http (client side) For Yourself
telnet panda.ece.utk.edu 80
open TCP connection to panda port 80, anything you type be sent to panda port 80 socket
Telnet to your favorite web site
Type in request, and look at the response
GET /~hqi/index.html HTTP/1.0
[hqi@com779 hqi]$ telnet panda.ece.utk.edu 80
Trying 160.36.30.108...
Connected to panda.ece.utk.edu.
Escape character is '^]'.
get /~hqi/index.html http/1.0
HTTP/1.1 501 Method Not Implemented
Date: Sun, 02 Sep 2001 21:03:28 GMT
Server: Apache/1.3.19 (Unix) (Red-Hat/Linux) PHP/4.0.4pl1
Allow: GET, HEAD, POST, PUT, DELETE, CONNECT, OPTIONS, PATCH, PROPFIND, PROPPATCH, MKCOL, COPY, MOVE, LOCK, UNLOCK, TRACE
Connection: close
Content-Type: text/html; charset=iso-8859-1
[hqi@com779 hqi]$ telnet panda.ece.utk.edu 80
Trying 160.36.30.108...
Connected to panda.ece.utk.edu.
Escape character is '^]'.
GET /~hqi/index.html http/1.0
HTTP/1.1 200 OK
Date: Sun, 02 Sep 2001 21:05:43 GMT
Server: Apache/1.3.19 (Unix) (Red-Hat/Linux) PHP/4.0.4pl1
Last-Modified: Sat, 07 Jul 2001 15:42:14 GMT
ETag: "1f222e-df9-3b472dd6"
Accept-Ranges: bytes
Content-Length: 3577
Connection: close
Content-Type: text/html
<HTML>
……
</HTML>
Connection closed by foreign host.
Add-on Features: Authentication
Purpose of authentication: control access to documentMeans: user name and passwordUser must present password on each request, Authorization:lineServer asks for it by giving it the response with WWW authenticate:
client server
usual http request msg401: authorization req.
WWW authenticate:
usual http request msg
+ Authorization:lineusual http response
msg
usual http request msg
+ Authorization:lineusual http response
msg
time
Add-on Feature: Cookies
Server sends user cookie in response message:
Set-cookie: 1678453
Client presents cookie in later request
cookie: 1678453
Server matches cookies with stored information: such as user preference, password etc
client server
usual http request msgusual http response
+Set-cookie: #
usual http request msg
cookie: #usual http response
msg
usual http request msg
cookie: #usual http response msg
cookie-spectificaction
cookie-spectificaction
Add-on Feature: Web Caches or Proxy Servers
Goal: to satisfy user request without invoking origin serverUser makes request, the object requested has been cached, then proxy server will reply, else proxy server request the object for client and then response
client
Proxyserver
client
http request
http re
quest
http response
http re
sponse
http re
quest
http re
sponse
http requesthttp response
origin server
origin server
Why Web Caching?
Cache should be closer to the clientsFaster responseReduce traffic (pay less money)
Web cache: Cost is low
originservers
public Internet
institutionalnetwork 10 Mbps LAN
1.5 Mbps access link
institutionalcache
FTP: File Transfer Protocol
transfer file to/from remote host client/server model
– client: initiates transfer (either to/from remote)– server: remote host
ftp: RFC 959
file transfer FTPserver
FTPuser
interface
FTPclient
local filesystem
remote filesystem
user at host
FTP: Operation Flowftp client contacts ftp server at port 21, specifying TCP as transport protocoltwo parallel TCP connections opened: control: exchange
commands, responses between client, server“out of band control”
data: file data to/from serverftp server maintains “state”: current directory, earlier authentication
FTPclient
FTPserver
TCP control connection
port 21
TCP data connectionport 20
Data connection is closed whenever it finished transferring one file.
FTP: Command and ResponseSample commands: sent as ASCII text over
control channel USER username PASS password LIST return list of file in
current directory RETR filename retrieves
(gets) file STOR filename stores
(puts) file onto remote host
Sample return codes status code and phrase
(as in http) 331 Username OK,
password required 125 data connection
already open; transfer starting
425 Can’t open data connection
452 Error writing file
Summary
Application Client Server Protocol
What type of service Through what interface Which port
DNS Aliasing vs. load distribution “dig” UDP
Email SMTP Mail access protocol
POP3 IMAP HTTP
TCP
Web http Stateless Persistent Pull vs. push Cookies, authentication, proxy
serverFile transfer
ftp Two connection flows Maintain state
Application protocols Push vs. pull State vs. stateless Persistent vs. non-persistent Port number Service model used